1. Field of the Invention
The present invention relates to a fixing device for use in a copier, printer, facsimile apparatus or similar image forming apparatus and more particularly to a transfixing type of fixing device configured to melt with heat a toner image transferred to a fixing roller at a secondary image transferring position and then transfer and fix the toner image to a sheet or recording medium by tertiary image transfer.
2. Description of the Background Art
A transfixing type of fixing device directly heats a toner image transferred to a fixing roller with a radiation source and a reflector substantially surrounding the radiation source. In this type of fixing device, when a toner image is transferred to the fixing roller, toner is directly heated by radiant heat in part of the fixing roller where the toner image is present while the surface of the fixing roller is directly heated in the other part where the toner image is absent. This brings about a problem that the temperature of the fixing roller becomes irregular along the image pattern after the toner image has been fixed on the sheet. As a result, when the next toner image is transferred to the fixing roller at the secondary image transfer position, the irregularity in temperature, corresponding to the previous image pattern, is likely to remain as a temperature history and effect the above toner image. Image defects include, e.g., irregularities in gloss and fixation and granularity.
A conventional fixing system applicable to an image forming apparatus fixes a toner image on a recording medium by heating and pressing the toner image. It is a common practice with such a fixing system to fix a toner image on a recording medium by melting the toner image with heat radiated from a halogen lamp or causing a heat-resistant film including a thin metallic layer or conductor to contact a recording medium implemented as an endless belt or a hollow cylinder. In any case, however, a fixing device generates a considerable amount of thermal energy which is likely to effect various units around the fixing device.
On the other hand, a heat pipe is capable of rapidly transporting, despite a small temperature difference, a great amount of thermal energy by use of latent heat derived from evaporation or condensation. A heat pipe is a heat conduction device in which a small amount of working liquid is sealed and having thermal conductivity several hundred times as high as the conductivity of copper, which is originally high.
Japanese patent laid-open publication Nos. 59-019976, 08-137277 and 2004-086091, for example, each disclose a particular image forming apparatus using a heat pipe as cooling means for protecting image forming sections other than heating means from heat output from the heating means. On the other hand, Japanese patent laid-open publication Nos. 60-154071 and 2000-29341, for example, propose to use a heat pipe as a fixing roller itself in order to reduce a warm-up time in combination with an induction heating effect, thereby realizing an on-demand fixing system that heats a fixing device only during intervals between consecutive printing.
Further, paying attention to the extremely high thermal conductivity of a heat pipe, Japanese patent laid-open publication No. 2001-201978, for example, proposes to use a heat pipe as a heat diffusing member for preventing, in a continuous print mode, heat from rising in the zones of a fixation nip outside of a sheet passing zone. Likewise, Japanese patent laid-open publication No. 2002-244450 proposes to use a heat pipe as temperature uniforming means for obviating temperature differences between a sheet passing portion and portions outside of the same.
Moreover, Japanese patent laid-open publication No. 2002-055552 teaches a fixing device of the type using a heat-resistant film whose thermal capacity is small and configured to reduce the irregular temperature distribution of the film in the widthwise direction. For this configuration, a heat pipe is included in a temperature irregularity reducing member positioned outside of a second roller body.